DE69611567T2 - Diesel engine with direct injection - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Technical field of the invention

The present invention relates to a diesel internal combustion engine with direct injection.

2. State of the art

In order to achieve high power output in an internal combustion engine, it is desirable that the average air-fuel ratio in the combustion chamber be made equal to the stoichiometric air-fuel ratio. The same applies to a diesel engine. However, in a diesel engine which directly injects fuel into the combustion chamber, if the average air-fuel ratio in the combustion chamber is made equal to the stoichiometric air-fuel ratio, the atomized fuel becomes extremely rich and a large amount of soot is generated. Accordingly, in the past, it was not possible for the air-fuel ratio in the combustion chamber of a diesel engine to be made equal to the stoichiometric air-fuel ratio, and so an excess of air was supplied to the combustion chamber (see, for example, Japanese Unexamined Published Utility Model (Kokai) No. 62-56743).

When fuel is injected into an intake port, the injected fuel is evenly distributed in the combustion chamber, so that even if the average air-fuel ratio in the combustion chamber is equal to the stoichiometric air-fuel ratio, no soot is produced, but in this case explosive combustion takes place. Accordingly, this technique cannot be adopted.

In the past, diesel engines had the problem that an excess of air had to be supplied to the combustion chamber and, as a result, the engine power output could not be improved.

GB-A-2 256 603 discloses a method of operating an internal combustion engine in which a fuel containing carbon and hydrogen, such as petrol, diesel fuel or alcohol, an oxygen-containing fuel is combusted and which comprises a catalyst arranged in the exhaust system at a distance from the engine and an afterburner and an ignition device upstream of the catalyst to assist in turning off the catalyst.

At start-up, an excess of fuel is supplied to the engine. This ensures that hydrogen is present in the exhaust gas. Oxygen is either present in the exhaust gas or is supplied via a pump. This combustible mixture is ignited by a device and heats the catalyst. Then, either the excess of fuel or the air supplied is regulated to produce a less flammable mixture which causes either stable combustion or a catalytic reaction.

EP-A-598 917 A1 discloses a system for controlling exhaust emission for a Internal combustion engine. A NOx absorbent is disposed in an exhaust passage of an internal combustion engine. This NOx absorbent absorbs NOx when the air-fuel ratio of exhaust gas flowing therein is lean, and discharges NOx which it has absorbed when the air-fuel ratio of exhaust gas flowing therein becomes rich. The amount of NOx absorbed in the NOx absorbent is estimated from the engine load and the rotation speed of the engine, and when this estimated NOx amount reaches the maximum NOx absorbing capacity of the NOx absorbent, the air-fuel ratio of exhaust gas flowing into the NOx absorbent is made rich.

In the article "Injection Of Methanol At Low Pressure In D. I. Dual-Fuel Engines" by P. Frederiksen, O. B. Nielsen and P. Sunn Pedersen of the paper 845126 of the XX FISITA CONGRESS entitled "The Automotive Future" in Vienna from 6 to 11 May 1984, on pages 4139 to 4142, a scientific study is disclosed in which methanol is directly injected into a diesel-gas engine at low pressures. The main objective of the study was to find out the maximum possible replacement of diesel fuel by methanol and the engine efficiencies and emissions. Strategies for load control of the diesel-gas engine and effects of increased intake pressure and increased intake temperature were also investigated.

In the article "Vegetable Oils and Alcohols Additive Fuels for Diesel Engines" by P. Tritthart and P. Zelenka of the paper 905112 of the XXIII FISITA CONGRESS entitled "The Promise Of New Technology In The Automotive Industry" in Turin, Italy, from May 7 to 11, 1990, on pages 849 to 855, it is disclosed that in the future, despite the present more than sufficient supply of crude oil, alternative fuels may be necessary to supplement gasoline and diesel fuels. Due to the increasing interest in the CO2- problem, the demand for alternative fuels from regenerated, vegetable energy sources is increasing. In such cases, it is also likely that the demand for energy sources with low emissions and low fuel consumption will increase. This would mean that the diesel engine, and in particular the direct injection diesel engine, has a great advantage over other combustion systems.

The work presented here describes experiments with rapeseed oil methyl ester and methanol in direct injection internal combustion engines for vehicle use. The main emphasis is on determining fuel consumption and exhaust emission potential in comparison with diesel operation, with special attention being given to emissions of polycyclic aromatic hydrocarbons and aldehydes, which are currently not taken into account.

SUMMARY OF THE INVENTION

The object of the present invention is to create a diesel internal combustion engine with direct injection in which the power output can be improved.

This object is solved by the subject matter of claim 1.

Further advantageous embodiments of the invention are the subject of the subclaims.

SHORT DESCRIPTION OF THE DRAWING

The present invention can be more fully understood from the description of the preferred embodiments of the invention set forth below, together with the accompanying drawings.

Show it:

Fig. 1 is an overall view of an internal combustion engine with internal combustion;

Figs. 2A and 2B are views showing the amount of injected fuel, the opening degree of an EGR control valve and an opening degree of a throttle valve;

Fig. 3A, 3B and 3C are illustrations of the amount of fuel injected, etc.;

Fig. 4 is a view showing changes of the feedback correction coefficient FAF;

Fig. 5 is a flow chart of the control by the feedback correction coefficient;

Fig. 6 is a flow chart of the control of the internal combustion engine operation;

Fig. 7 is a flow chart of another embodiment for controlling engine operation;

Fig. 8 is an overall view of another embodiment of the internal combustion engine;

Fig. 9 is a view showing the rate A of excess air, i.e. the target air-fuel ratio (A/F)₀;

Fig. 10 is a graphical representation of the opening degrees of the EGR control valve and the throttle valve;

Fig. 11A and 11B are illustrations of the opening degree G of the EGR control valve and the opening degree 6 of the throttle valve;

Fig. 12 is a graphical representation of the output of the air-fuel ratio sensor;

Figs. 13A and 13B are views for explaining the absorption and discharge operation of a NOx absorbent;

Fig. 14A, 14B and 14C are illustrations of the amount A of NOx absorption, etc.;

Fig. 15 is a time chart of the changes in the estimated absorbed NOx, i.e., ΣNOx;

Fig. 16 is a timing chart of the control in which NOx is discharged; and

Fig. 17 and 18 Flow charts of the operation control.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to Fig. 1, reference numeral 1 is an internal combustion engine of the direct injection type, reference numeral 2 is a piston, reference numeral 3 is a combustion chamber, reference numeral 4 is a fuel injector for directly injecting fuel into the combustion chamber 3, reference numeral 5 is an intake valve, reference numeral 6 is an intake port, reference numeral 7 is an exhaust valve, and reference numeral 8 is an exhaust port. The intake port 6 is connected to an expansion tank 10 by a corresponding intake branch 9, while the expansion tank 10 is connected to an air cleaner 12 by an intake pipe 11. A throttle valve 14 is provided in the intake pipe 11 and is controlled to open and close by a drive motor 13. On the other hand, the exhaust channel 8 is connected by an exhaust manifold 15 and an exhaust pipe 16 to a catalyst system 18 in which a three-way catalyst 17 is arranged.

The exhaust manifold 15 and the surge tank 10 are connected to each other through an exhaust gas recirculation (EGR) passage 19. In this EGR passage 19, an EGR control valve 20 is arranged to regulate the amount of EGR gas which is returned from the exhaust manifold 15 to the surge tank 10. On the other hand, the fuel injector 4 is connected to a fuel injection pump 21 which is driven by the internal combustion engine. The fuel injected from the fuel injection pump 21 is supplied to the fuel injector 4. This fuel injection pump 21 is controlled to feed based on the output signal from an electronic control unit 30, and therefore the injection amount from the fuel injector 4 is controlled. 4 is controlled based on the output signal of the electronic control unit 30.

The electronic control unit 30 comprises a digital computer which has a read-only memory (ROM) 32, a random access memory (RAM) 33, a microprocessor (CPU) 34, an input terminal 35 and an output terminal 36 which are connected to each other by a bidirectional bus 31. In the exhaust manifold 15, an air-fuel ratio sensor (hereinafter also referred to as O2-sensor) is arranged to detect the average air-fuel ratio in the combustion chamber 3 from the concentration of oxygen in the exhaust gas. The output signal of the O2-sensor is supplied to the input terminal 35 through a corresponding A/D converter 37. In addition, an accelerator pedal 23 is connected to a load sensor 24 which generates an output voltage proportional to the degree to which the accelerator pedal 23 is depressed. The output voltage of the load sensor 24 is supplied to the input terminal 35 through a corresponding A/D converter 37. In addition, the input terminal 35 is connected to an engine speed sensor 25 which generates an output pulse indicative of the engine speed. On the other hand, the output terminal 36 is connected to the drive motor 13, the EGR control valve 20 and the fuel injection pump 21 through a corresponding control circuit 38.

In the embodiment shown in Fig. 1, oxygen-containing fuel is used as the fuel injected from the fuel injector 4 into the interior of the combustion chamber 3. In addition, the average air-fuel ratio in the combustion chamber 3 is controlled on the substantially stoichiometric air-fuel ratio. In this case, as the oxygen-containing fuel, either fuel containing oxygen atoms in the molecules themselves or fuel obtained by adding an oxygen-containing additive can be used. It does not matter what kind of fuel is used. The fuel injected from the fuel injector 4 ultimately contains oxygen.

Fig. 2A shows the relationship between the injection amount Q of oxygen-containing fuel and the depression amount L of the accelerator pedal 23 in the case of a constant engine speed. According to Fig. 2A, the larger the depression amount L of the accelerator pedal 23, that is, the heavier the engine load, the larger the injection amount Q of fuel becomes. It should be noted that in practice, the injection amount Q of fuel is not only a function of the depression amount L of the accelerator pedal but also of the engine speed N. Accordingly, the injection amount Q of fuel is stored in advance in the ROM 32 in the form of the map shown in Fig. 3A.

On the other hand, in order to keep the average air-fuel ratio in the combustion chamber 3 substantially at the stoichiometric air-fuel ratio, the method may be used in which the recirculated amount of the EGR gas is increased and the amount of intake air supplied to the combustion chamber 3 is reduced as the engine load is lower, or the method may be used in which the opening degree of the throttle valve 14 and the amount of intake air supplied to the combustion chamber 3 are reduced as the engine load is lower. In addition, these methods are applied simultaneously. Fig. 2B shows the case where these methods are applied simultaneously. In this case, as shown in Fig. 2B, the smaller the depression amount L of the accelerator pedal 23 is, the larger the opening degree of the EGR control valve 20 is made, that is, the more the amount of EGR gas is increased, and at the same time, the smaller the opening degree of the throttle valve 14 is made. It is to be noted that in practice, the opening degree of the EGR control valve 20 and the opening degree of the throttle valve 14 are functions of not only the depression amount L of the accelerator pedal 23 but also the engine speed N. Accordingly, the opening degree θ of the EGR control valve 20 and the opening degree θ of the throttle valve 14 are stored in advance in the ROM 32 in the form of the maps shown in Figs. 3B and 3C.

Accordingly, based on the depression amount L of the accelerator pedal 23 and the engine speed N, when the value shown in Fig. 3A is made equal to the fuel injection amount Q, the value shown in Fig. 3B is made equal to the opening degree θ of the EGR control valve 20, and the value shown in Fig. 3C is made equal to the opening degree θ of the throttle valve 14, the average air-fuel ratio in the combustion chamber 3 becomes substantially the stoichiometric air-fuel ratio. In this case, if no oxygen is contained in the fuel, the fuel injected from the fuel injector 4 becomes extremely rich in the vaporized portion, and therefore a large amount of soot is generated. However, in the present invention, oxygen is contained in the fuel, so that the oxygen is evenly distributed in the vaporized fuel, and therefore, in the presence of sufficient oxygen, the fuel particles are burned and excellent combustion which is not accompanied by the generation of soot can be achieved. It is to be noted that the opening degree G of the EGR control valve 20 shown in Fig. 3A and the opening degree θ of the throttle valve 14 shown in Fig. 3C are naturally set such that the air-fuel ratio in the combustion chamber 3 becomes the stoichiometric air-fuel ratio after taking into account the amount of oxygen contained in the fuel.

In this way, in the embodiment shown in Fig. 1, the air-fuel ratio in the combustion chamber 3 can be maintained substantially at the stoichiometric air-fuel ratio, so that it is possible to achieve high engine output.

However, when the content of oxygen in the fuel differs from the target content determined in advance, when Q, G and θ are determined from the maps shown in Figs. 3A, 3B and 3C, the average air-fuel ratio in the combustion chamber 3 ends up deviating from the target air-fuel ratio (A/F)₀. Therefore, in the embodiment shown in Fig. 1, the fuel injection amount Q or the amount of EGR gas is feedback-controlled based on the output signal of the O2⁻ sensor 22 so that the average air-fuel ratio in the combustion chamber 3 becomes the stoichiometric air-fuel ratio. It should be noted that performing feedback control in this manner also enables the purification effect of the three-way catalyst 17 with respect to NOx, HC and CO to be improved.

Next, the feedback control will be explained. The O2- sensor 22 produces an output voltage V of about 0.1 V as shown in Fig. 4 when the average air-fuel ratio in the combustion chamber 3 is lean, and produces an output voltage V of about 0.9 V when the average air-fuel ratio in the combustion chamber 3 is rich. When feedback control is performed on the fuel injection amount Q, the fuel injection amount Q is corrected by the feedback correction coefficient FAF. This feedback correction coefficient FAF is controlled based on the output voltage V of the O2- sensor 22 as shown in Fig. 4.

Fig. 5 shows the routine for controlling the feedback correction coefficient FAF based on the output voltage V of the O2- sensor 22. This routine is performed while interrupting at every predetermined time interval. Referring now to Fig. 5, first, in step 100, it is determined whether the output voltage V of the O2- sensor 22 is higher than a reference value Vr (Fig. 4). If V > Vr, that is, if the ratio is rich, the routine proceeds to step 101 where it is determined whether the ratio was lean at the time of the previous interruption. If it was lean at the time of the previous interruption, the routine proceeds to step 102 where a skip value S is subtracted from the feedback correction coefficient FAF. In contrast, if the ratio was rich at the time of the previous interruption, the routine proceeds to step 103, where the integration value K (K << S) is subtracted from the FAF. Accordingly, as shown in Fig. 4, the FAF is rapidly decreased by the skip value S and then gradually decreases as the ratio changes from lean to fat.

On the other hand, if it is determined in step 100 that V ≤ Vr, that is, if it is determined that the ratio is lean, the routine proceeds to step 104 where it is determined whether the ratio was rich at the time of the previous interruption. If it was rich at the time of the previous interruption, the routine proceeds to step 105 where the skip value S is added to the FAF. In contrast, if the ratio was lean at the time of the previous interruption, the routine proceeds to step 106 where the integration value K is added to the FAF. Accordingly, as shown in Fig. 4, the FAF is rapidly increased by the skip value S and then gradually increased as the ratio changes from rich to lean.

Fig. 6 shows the routine for controlling the operation of the internal combustion engine. This routine is repeatedly executed.

Referring to Fig. 6, first, in step 150, the fuel injection amount Q is calculated from the map shown in Fig. 3A. Then, in step 151, the opening degree G of the EGR control valve 20 is calculated, and the opening degree of the EGR control valve 20 is controlled to this opening degree G. Then, in step 152, the opening degree θ of the throttle valve 14 is calculated from the map shown in Fig. 3C, and the drive motor 13 is driven so that the throttle valve 14 reaches this opening degree θ. Then, in step 153, the feedback correction coefficient FAF is multiplied by the fuel injection amount Q to obtain the final fuel injection amount. Q and the fuel injection pump 21 is controlled such that it can inject this quantity Q.

Fig. 7 shows the routine for controlling the operation of the internal combustion engine in the case where the amount of EGR gas is controlled so that the average air-fuel ratio in the combustion chamber 3 becomes the stoichiometric air-fuel ratio. Note that in this case, the routine shown in Fig. 5 is also used to calculate the feedback correction coefficient FAF.

Referring to Fig. 7, first, in step 200, the fuel injection amount Q is calculated from the map shown in Fig. 3A, and the fuel injection pump 21 is controlled so that it can inject this amount Q. Then, in step 201, the opening degree G of the EGR control valve 20 is calculated from the map shown in Fig. 3B. Then, in step 202, the opening degree θ of the throttle valve 13 is calculated from the map shown in Fig. 3C, and the drive motor 13 is driven so that the throttle valve 14 reaches this opening degree θ. Then, in step 203, the feedback correction coefficient FAF is multiplied by the opening degree G of the EGR control valve 20 to determine the final opening degree G of the EGR control valve 20. The opening degree of the EGR control valve 20 is controlled to this opening degree G.

Fig. 8 to 18 show another embodiment. When an oxygen-containing fuel is used in the above manner, even if the air-fuel ratio is kept at the stoichiometric air-fuel ratio, is maintained, excellent combustion can be achieved which is not accompanied by the generation of soot. However, in some types of direct injection diesel engines, problems occur if the air-fuel ratio is maintained at the stoichiometric air-fuel ratio when the engine is operating under a light load or a heavy load. That is, when the engine is operating under a light load, a large amount of EGR gas is recirculated so that the amount of air in the combustion chamber 3 becomes small, and therefore, if the air-fuel ratio is maintained at the stoichiometric air-fuel ratio at this time, there is not enough air even when oxygen-containing fuel is used, and thus the combustion becomes unstable.

When the engine is operated under a heavy load with a large amount of fuel being injected, the uneven distribution of the injected fuel causes an excessively rich air-fuel mixture region to be formed in the combustion chamber 3, and therefore, at that time, if the air-fuel ratio is made equal to the stoichiometric air-fuel ratio, even if an oxygen-containing fuel is used, there will be insufficient air and soot will be generated. Accordingly, it is necessary to make the air-fuel ratio lean when the engine is operated under a light load or under a heavy load in an internal combustion engine of this type. Figs. 8 to 18 show an embodiment suitable for this type of internal combustion engine.

First, referring to Fig. 8 in this embodiment, the housing 27 in which the NOx absorber 26 is arranged is arranged in the engine exhaust passage upstream of the catalyst system 18 in which the three-way catalyst 17 is arranged.

Fig. 9 shows the relationship between the depression amount L of the accelerator pedal 23, the engine speed N, and the target excess air rate λ, i.e., the target air-fuel ratio (A/F)₀⁻. According to Fig. 9, when the engine is operating under a medium load at a medium speed, the target excess air rate λ is made 1.0, i.e., the target air-fuel ratio (A/F)₀ is made equal to the stoichiometric air-fuel ratio. In other areas, the target excess air rate A is made greater than 1.0, i.e., the target air-fuel ratio (A/F)₀ is made lean.

On the other hand, in order to keep the excess air rate λ at the target excess air rate shown in Fig. 9, that is, to keep the average air-fuel ratio in the combustion chamber 3 at the target air-fuel ratio (A/F)₀, the method may be adopted in which the amount of recirculated EGR gas is increased and the amount of intake air supplied to the combustion chamber 3 is reduced as the engine load is lower, or the method may be adopted in which the opening degree of the throttle valve 14 and the amount of intake air supplied to the combustion chamber 3 are reduced as the engine load is lower. In addition, these methods may be adopted simultaneously. The solid line in Fig. 10 shows the Case where these methods are simultaneously applied. In this case, as shown by the solid line in Fig. 10, the smaller the depression amount L of the accelerator pedal 23 is, the larger the opening degree of the EGR control valve 20 is made, that is, the more the amount of EGR gas is increased, and at the same time, the smaller the opening degree of the throttle valve 14 is made. It is to be noted that, in practice, the opening degree of the EGR control valve 20 and the opening degree of the throttle valve 14 are not only functions of the depression amount L of the accelerator pedal 23 but also functions of the engine speed N. Accordingly, the opening degree G of the EGR control valve 20 and the opening degree θ of the throttle valve 14 are stored in advance in the ROM 32 in the form of the maps shown in Figs. 11A and 11B.

Accordingly, based on the depression amount L of the accelerator pedal 23 and the engine speed N, when the value shown in Fig. 3a is made the fuel injection amount Q, the value shown in Fig. 11A is made the opening degree of the EGR control valve 20, and the value shown in Fig. 11B is made the opening degree θ of the throttle valve 14, the average air-fuel ratio in the combustion chamber 3 becomes the target air-fuel ratio (A/F)₀. Note that the opening degree G of the EGR control valve 20 shown in Fig. 11A and the opening degree θ of the throttle valve 14 shown in Fig. 11B are set so that the air-fuel ratio in the combustion chamber 3 becomes the target air-fuel ratio (A/F)₀. after taking into account the amount of oxygen contained in the fuel.

However, if the amount of oxygen in the fuel is different from the predetermined amount, then when Q, G and θ are determined from the graphs shown in Figs. 3A, 11A and 11B, the average air-fuel ratio in the combustion chamber 3 ends up deviating from the target air-fuel ratio (A/F)₀. Therefore, in this embodiment too, the fuel injection amount Q is feedback-controlled based on the output signal of the air-fuel ratio sensor 22 so that the average air-fuel ratio in the combustion chamber 3 becomes the target air-fuel ratio (A/F)₀.

Next, the feedback control will be explained in a simple manner. The air-fuel ratio sensor 22 shown in Fig. 8 has characteristics different from the O2- sensor shown in Fig. 1. Fig. 12 shows the relationship between the output voltage V of the air-fuel ratio sensor 22 shown in Fig. 8 and the average air-fuel ratio A/F in the combustion chamber 3. From Fig. 12, it is learned that when the air-fuel ratio sensor 22 shown in Fig. 8 is used, the average air-fuel ratio A/F in the combustion chamber 3 can be detected. In the embodiment shown in Fig. 8, when the average air-fuel ratio A/F detected by the air-fuel ratio sensor 22 is larger than the target air-fuel ratio (A/F)₀, the fuel injection amount Q is increased, while when the average air-fuel ratio A/F detected by the air-fuel ratio sensor 22 is smaller than the target air-fuel ratio (A/F)₀, the fuel injection amount Q is Q is reduced. The average air-fuel ratio A/F is thus controlled to the target air-fuel ratio (A/F)₀.

That is, in this embodiment, when the internal combustion engine is operating at a medium speed under a medium load, the average air-fuel ratio A/F is feedback-controlled to the stoichiometric air-fuel ratio. At this time, the unburned HC, CO and NOx contained in the exhaust gas are effectively removed by the three-way catalyst 17. On the other hand, when the internal combustion engine is operating at a speed other than the medium speed and a load other than the medium load, the average air-fuel ratio A/F is feedback-controlled to the target air-fuel ratio (A/F)0. If the average air-fuel ratio A/F is kept lean, a large amount of NOx is generated, but this NOx is absorbed by the NOx absorber 26. Therefore, this NOx absorption device 26 will be explained below.

The NOx absorption device 16 arranged in the housing 27 consists, for example, of a carrier made of aluminum oxide, on which, for example, a noble metal such as platinum Pt and at least one element selected from the group of alkali metals such as potassium K, sodium Na, lithium Li and cesium Cs, alkaline earths such as barium Ba and calcium Ca, and rare earths such as lanthanum La and yttrium Y are held. If the ratio of the air and the fuel supplied to the engine intake port, the combustion chamber 3 and the exhaust port upstream of the NOx absorption device 26 is defined as the air-fuel ratio of the inflowing exhaust gas flowing into the NOx absorber 26, the NOx absorber 26 absorbs the NOx when the air-fuel ratio of the inflowing exhaust gas is lean, and discharges the absorbed NOx when the concentration of oxygen in the inflowing exhaust gas falls, that is, it performs a NOx absorbing and a NOx discharging operation. It is to be noted that when the fuel or air is not supplied to the exhaust passage upstream of the NOx absorbent 26, the air-fuel ratio of the inflowing exhaust gas matches the average air-fuel ratio in the combustion chamber 3, and therefore, in this case, the NOx absorbent 26 absorbs the NOx when the average air-fuel ratio in the combustion chamber 3 is lean, and discharges the absorbed NOx when the concentration of oxygen in the combustion chamber 3 falls.

By arranging the above NOx absorber 26 in the engine exhaust passage, the NOx absorber 26 actually works for the purpose of absorbing and discharging NOx, but there are portions of the detailed mechanism of this absorption and discharge action that are not clear. However, it is expected that this absorption and discharge action takes place due to the mechanism shown in Figs. 13A and 13B. This mechanism will be explained next, taking as an example the case where platinum Pt and barium Ba are supported on the carrier, but the same mechanism works when other noble metals, alkali metals, alkaline earths and rare earths are used.

When this average air-fuel ratio A/F in the combustion chamber 3 is kept lean, the concentration of oxygen in the inflowing exhaust gas is high. Therefore, at this time, as shown in Fig. 13A, the oxygen O₂ is deposited on the surface of the platinum Pt in the form of CO2⁻ or O2⁻. On the other hand, the NO in the inflowing exhaust gas reacts with the O2⁻ or the O2⁻ on the surface of the platinum Pt to give NO₂ (2NO + O₂ → 2NO₂). Then, a part of the generated NO₂ is oxidized on the platinum Pt and absorbed in the absorber, where it is combined with the barium oxide BaO and absorbed in the absorber in the form of nitrate ions NO3⁻. dispersed as shown in Fig. 13A. In this way, the NOx is absorbed in the NOx absorber 26.

NO₂ is generated on the surface of the platinum Pt as long as the concentration of oxygen in the incoming exhaust gas is high. NO₂ is absorbed in the absorber as long as the concentration of oxygen in the incoming exhaust gas and the amount of NO₂ generated fall, the reaction proceeds in the opposite direction (NO₃⁻ → NO₂), and therefore nitrate ions NO₃⁻ are discharged from the absorber in the form of NO₂ in the absorber. That is, as the concentration of oxygen in the incoming exhaust gas falls, NOx is discharged from the NOx absorber 26. Therefore, as the degree of leanness of the incoming exhaust gas falls, the concentration of oxygen in the incoming exhaust gas falls. Therefore, when the degree of leanness of the incoming exhaust gas falls, the NOx Absorption device 26 emits NOx even when the air-fuel ratio of the incoming exhaust gas is lean.

On the other hand, when the average air-fuel ratio in the combustion chamber 3 is made rich and the air-fuel ratio of the inflowing exhaust gas becomes rich at this time, a large amount of unburned HC and CO are discharged from the internal combustion engine. These unburned HC and CO react with the oxygen CO₂⁻ or O⁻ on the platinum Pt so that they are oxidized. In addition, when the air-fuel ratio of the inflowing exhaust gas becomes rich, the concentration of oxygen in the inflowing exhaust gas drops sharply so that NO₂ is discharged from the absorbent. This NO₂ is reduced by a reaction with the unburned HC and the unburned CO as shown in Fig. 13B. When NO₂ no longer exists on the surface of the platinum Pt, the NO₂ is oxidized. discharged from the absorber. Accordingly, when the air-fuel ratio of the inflowing exhaust gas is made rich, the NOx is discharged from the NOx absorber 26 in a short time.

That is, when the air-fuel ratio of the incoming exhaust gas is made rich, firstly, the unburned HC and CO immediately react with the O₂⁻ or the O²⁻ on the platinum Pt so as to be oxidized. If the unburned HC or CO still remains even after O₂⁻ or O²⁻ on the platinum Pt is consumed, then the NOx discharged from the absorber and the NOx discharged from the internal combustion engine are reduced. Accordingly, by making the air-fuel ratio of the incoming exhaust gas rich, the NOx absorbed in the NOx absorbent 26 is discharged in a short time, and furthermore, the discharged NOx is reduced, so that discharge of NOx into the atmosphere can be prevented. In addition, because the NOx absorbent 26 has a function of a reduction catalyst, the NOx discharged from the NOx absorbent 26 can be reduced even when the air-fuel ratio of the inflowing exhaust gas is made equal to the stoichiometric air-fuel ratio. However, when the air-fuel ratio of the inflowing exhaust gas is made equal to the stoichiometric air-fuel ratio, the NOx is discharged from the NOx absorbent 26 only gradually, so that a somewhat longer period of time is required to discharge all of the NOx absorbed in the NOx absorbent 26.

According to the above explanation, when the average air-fuel ratio A/F in the combustion chamber 3 is kept lean, the NOx continues to be absorbed in the NOx absorbent 26. However, there is a limit to which the NOx absorbent 26 can absorb NOx. When the NOx absorbent 26 reaches its limit for absorbing NOx, the NOx absorbent 26 can no longer absorb NOx. Accordingly, it is necessary to discharge the NOx from the NOx absorbent 26 before the NOx absorbent 26 reaches its limit for absorbing NOx. Therefore, it is necessary to estimate to what extent the NOx has been absorbed in the NOx absorbent 26. This method of estimating the amount of NOx absorbed will be briefly explained below.

When the average air-fuel ratio in the combustion chamber 3 is kept lean, the higher the engine load becomes, the more NOx emitted from the engine per unit time becomes, so that the NOx absorbed in the NOx absorbent 26 per unit time becomes more. In addition, the higher the engine speed becomes, the more NOx emitted from the engine per unit time becomes, so that the NOx absorbed in the NOx absorbent 26 per unit time becomes more. Accordingly, the amount of NOx absorbed in the NOx absorbent 26 per unit time becomes a function of the engine load and the engine speed. Accordingly, in the embodiment shown in Fig. 8, the amount A of NOx absorbed in the NOx absorbent 26 per unit time is determined in advance by experiments as a function of the depression amount L of the accelerator pedal 23 and the engine speed N. The amount A of NOx is stored in advance in the ROM 32 in the form of the map shown in Fig. 14A as a function of L and N.

On the other hand, when the average air-fuel ratio A/F becomes equal to the stoichiometric air-fuel ratio or becomes rich, NOx is discharged from the NOx absorber 26. At this time, the NOx discharge D discharged from the NOx absorber 26 per unit time is proportional to the amount of exhaust gas and the richness of the average air-fuel ratio A/F. In this case, the amount of exhaust gas is a function of the depression amount L of the accelerator pedal 23 and the engine speed N, so that the NOx discharge D also becomes a function of the depression amount L of the accelerator pedal 23 and the engine speed N. The NOx discharge D is stored in the ROM 32 in the form shown in Fig. 14B. shown in the figure. On the other hand, when the richness degree of the average air-fuel ratio A/F becomes higher, the NOx discharge rate K becomes higher as shown in Fig. 14C, and therefore, when the NOx discharge rate K is taken into consideration, the NOx discharge per unit time is expressed by K·D.

Since the NOx absorption per unit time by A and the NOx discharge per unit time by K D are expressed in this way, the NOx estimated to be absorbed in the NOx absorbent 26, i.e., ΣNOx, is expressed by the following equation:

ΣNOx = ΣNOx + A - K D

Fig. 15 shows the changes in the estimated absorbed NOx, i.e., ΣNOx. According to Fig. 15, when the average air-fuel ratio A/F is lean, the estimated absorbed NOx, i.e., ΣNOx, gradually increases. When the average air-fuel ratio A/F is maintained at the stoichiometric air-fuel ratio, the estimated absorbed NOx, i.e., ΣNOx, is gradually reduced. In addition, when the estimated absorbed NOx, i.e., ΣNOx, exceeds a predetermined upper limit MAX, the average air-fuel ratio A/F is made rich for a short period like a peak where the NOx is to be discharged from the NOx absorbent 26.

In this way, in the embodiment shown in Fig. 8, when the NOx is to be discharged from the NOx absorbent 26, the average air-fuel ratio A/F in the Combustion chamber 3 is made rich. However, in the present invention, the oxygen is contained in the fuel, so that oxygen is uniformly distributed in the vaporized fuel and accordingly the fuel particles burn in the presence of a large amount of oxygen so that only a small amount of soot is generated. That is, by using an oxygen-containing fuel in this manner, the average air-fuel ratio in the combustion chamber 3 can be made rich for the first time.

However, when the fuel injection amount is just controlled to make the average air-fuel ratio in the combustion chamber 3 rich, that is, when the average air-fuel ratio is made rich by just increasing the fuel injection amount, the output torque increases rapidly and a shock is generated. Therefore, in the embodiment shown in Fig. 8, the opening degree of the EGR control valve 20 is made larger and the opening degree of the throttle valve 14 is made smaller, and furthermore the fuel injection amount is increased so that the average air-fuel ratio A/F is made rich. Fig. 16 shows the changes in the average air-fuel ratio A/F, etc., at this time. It is note that the dashed line in Fig. 10 shows the opening degree G' of the EGR control valve 20 and the opening degree θ' of the throttle valve 14 at this time.

Fig. 17 and Fig. 18 show the routine for controlling the operation of the internal combustion engine. The routine is executed, for example, interrupted at certain time intervals.

Referring to Figs. 17 and 18, First, in step 250, the fuel injection amount Q is calculated from the map shown in Fig. 3A. Next, in step 251, it is determined whether or not the NOx discharge flag, which is set when the NOx is to be discharged from the NOx absorbent 26, is set. If the NOx discharge flag is not set, the routine proceeds to step 252, where the opening degree G of the EGR control valve 20 is calculated from the map shown in Fig. 11A. Next, in step 253, the opening degree θ of the throttle valve 14 is calculated from the map shown in Fig. 11B.

Next, in step 254, it is determined whether the average air-fuel ratio A/F in the combustion chamber 3 detected by the air-fuel ratio sensor 22 is greater than the target air-fuel ratio (A/F)₀. If A/F > (A/F)₀, the routine proceeds to step 255 where a predetermined value K is added to the feedback correction coefficient FAF. Then, the routine proceeds to step 257. On the contrary, if A/F ≤ (A/F)₀, the routine proceeds to step 256 where the predetermined value K is subtracted from the feedback correction coefficient FAF. Then, the routine proceeds to step 257. In step 257, the feedback correction coefficient FAF is multiplied by the fuel injection amount Q so that the final fuel injection amount Q (= FAF·Q) is calculated. That is, when A/F > (A/F)₀, the fuel injection amount Q is reduced, thereby maintaining the average air-fuel ratio A/F at the target air-fuel ratio (A/F)₀. It It should be noted that FAF moves above and below the value of 1.0.

Then, in step 258, it is determined whether the target air-fuel ratio (A/F)₀ is the stoichiometric air-fuel ratio. If the target air-fuel ratio (A/F)₀ is not the stoichiometric air-fuel ratio, that is, if the target air-fuel ratio (A/F)₀ is lean, the routine proceeds to step 259, where the NOx absorption A shown in the map of Fig. 14A is added to the estimated absorbed NOx, that is, ΣNOx. Then, the routine proceeds to step 263. On the contrary, if the target air-fuel ratio (A/F)₀ is lean, the routine proceeds to step 264. is the stoichiometric air-fuel ratio, the routine proceeds to step 260 where the product of K D of the NOx discharge D determined from Fig. 14B and the NOx discharge rate K determined from Fig. 14C based on the average air-fuel ratio (14.7 Q/(Q + ΔQ)) is subtracted from the estimated absorbed NOx, i.e., ΣNOx. Then, in step 261, it is determined whether or not the estimated absorbed NOx, i.e., ΣNOx, has become negative. If Z NOx < 0, then the routine proceeds to step 262 where the estimated absorbed NOx, i.e., ΣNOx, is made zero. The routine then continues with step 263.

In step 263, it is determined whether the estimated absorbed NOx, ie, ΣNOx, has exceeded the upper limit MAX. If ΣNOx ≤ MAX, the routine ends. On the contrary, if ΣNOx > MAX, the routine proceeds to step 264, where the NOx purge flag is set. If the NOx purge flag is set, the routine proceeds to the The next pass from step 251 proceeds to step 265 where the discharge of NOx from the NOx absorber 26 is controlled.

That is, in step 265, the opening degree G' of the EGR control valve 20 shown in Fig. 10 was calculated, and the EGR control valve 20 was opened to this opening degree G'. Then, in step 266, the opening degree θ' of the throttle valve 14 shown in Fig. 10 was calculated, and the throttle valve 14 was closed to this opening degree θ'. Then, in step 267, it is judged whether a predetermined period of time has elapsed from the start of the operation for opening the EGR control valve 20 and the operation for closing the throttle valve 14. If a predetermined period of time has elapsed, the routine proceeds to step 268.

In step 268, the fuel injection amount Q is increased by exactly ΔQ, thereby making the average air-fuel ratio A/F rich. Then, in step 269, the product K·D of the NOx discharge Dr obtained from Fig. 14B and the NOx discharge rate K obtained from Fig. 14C is subtracted from the estimated absorbed NOx, ie, ΣNOx, based on the average air-fuel ratio (14.7·Q/(Q + ΔQ)). Then, in step 270, it is determined whether or not the estimated absorbed NOx, ie, ΣNOx, has become negative. When Z NOx < 0, that is, when all of the NOx has been discharged from the NOx absorbent 26, the routine proceeds to step 271, where the estimated absorbed NOx, that is, ΣNOx, is made zero. Then, the routine proceeds to step 272, where the NOx discharge flag is reset. When the NOx discharge flag is reset, in the next run, the EGR control valve 20 is controlled to the opening degree G closed and the throttle valve 14 is opened to the opening degree θ.

It is to be noted that in the embodiment shown in Fig. 8, the NOx absorbent 26 is arranged upstream of the three-way catalyst 17. When the NOx absorbent 26 is arranged upstream of the three-way catalyst 17 in this manner, it is advantageous in that the NOx which could not be reduced in the NOx absorbent 26 at the time of discharging NOx from the NOx absorbent 26 can be reduced by the three-way catalyst.

Claims (9)

1. Diesel internal combustion engine, with a combustion chamber (3), with: a fuel injection device (4, 21) which, in use, injects fuel containing oxygen into the combustion chamber (3); and means for determining an amount of fuel injected by the fuel injection means so that an average value of an air-fuel ratio in the combustion chamber (3) becomes equal to a target air-fuel ratio which is either the stoichiometric air-fuel ratio or a lean air-fuel ratio, wherein an air-fuel ratio sensor (22) is arranged in an engine exhaust passage (15), and wherein an air-fuel ratio control device is provided for controlling an air-fuel ratio in the combustion chamber (3) to a target air-fuel ratio based on an output signal of the air-fuel ratio sensor (22), and wherein the air-fuel ratio is controlled to become the target air-fuel ratio by controlling at least an amount of recirculated exhaust gas. 2. Diesel internal combustion engine according to claim 1, in which an exhaust gas recirculation control device (19) is provided is to regulate an amount of exhaust gas recirculated from the engine exhaust port (15) to an engine intake port (9), and in which the amount of recirculated exhaust gas is greater the lower the engine load is. 3. Diesel engine according to claim 2, in which a throttle valve (14) is provided in the engine intake port (9), and in which the opening degree of the throttle valve (14) is made smaller the lower the engine load is. 4. Diesel internal combustion engine according to one of the preceding claims, in which a three-way catalyst (17, 18) is arranged in the internal combustion engine outlet channel (15). 5. Diesel internal combustion engine according to one of the preceding claims, in which a feedback air-fuel ratio control device is provided to carry out feedback control of the air-fuel ratio in the combustion chamber (3) to a target air-fuel ratio on the basis of an output signal of the air-fuel ratio sensor (22). 6. A diesel engine according to claim 1, wherein a NOx absorber (26) is arranged in the engine exhaust passage (15) to absorb NOx when an air-fuel ratio of exhaust gas flowing into the NOx absorber (26) is lean, and to discharge the absorbed NOx when the air-fuel ratio of the exhaust gas flowing into the NOx absorber (26) is stoichiometric. 7. Diesel internal combustion engine according to claim 4 and 6, in which the three-way catalyst (17, 18) is arranged downstream of the NOx absorption device (26). 8. Diesel engine according to one of the preceding claims, in which the amount of fuel injection is increased when the average air-fuel ratio in the combustion chamber (3) is switched from lean to rich in order to discharge NOx from the NOx absorber (26). 9. A diesel engine according to any one of the preceding claims, in which the opening degree of the throttle valve (14) is made smaller when the average air-fuel ratio in the combustion chamber (3) is switched from lean to rich in order to discharge NOx from the NOx absorber.